You cannot push on the world without the world pushing back, equally hard and in the exact opposite direction. A swimmer pushes water back and the water drives them forward; a rocket throws gas down and the gas throws the rocket up. Every force comes in a pair. Learn to name both members of the pair and you will never confuse "equal and opposite" forces with forces that cancel.
A small car and a large truck collide head-on. During the crash, each pushes on the other.
Before reading: which experiences the larger collision force, the car, the truck, or are the two forces equal? Write your prediction and one sentence of reasoning.
If A pushes B, then B pushes A, equally and oppositely
A force is never a one-way action: whenever one object pushes or pulls another, the second object pushes or pulls back with exactly the same strength in the opposite direction.
Newton's third law states that for every action force there is an equal and opposite reaction force. The two forces are always the same type of interaction (both gravitational, both contact, etc.), equal in magnitude, opposite in direction, and act on two different objects. We can write this as:
$$F_{A\,\text{on}\,B} = -\,F_{B\,\text{on}\,A}$$
Newton's third law: if A exerts a force on B, then B exerts an equal and opposite force on A ($F_{A\,\text{on}\,B} = -F_{B\,\text{on}\,A}$). The two forces are equal in size, opposite in direction, and act on different objects, so they never cancel on a single object.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: the two forces in an action–reaction pair act on the same object and therefore cancel.
The pair is equal in force, but not in the effect on each object
We just saw that the action and reaction forces are equal in size. That raises a question: if the forces are equal, why does a gun barely move while the bullet flies off fast? This card answers it → equal forces produce different accelerations because the masses differ ($a = F/m$).
Equal-and-opposite does not mean equal effect: the same force gives a small mass a big acceleration and a large mass a tiny one.
When two objects interact, each feels the same-sized force, but each object's acceleration follows the second law applied to its own mass, $a = F/m$. A small mass therefore accelerates much more than a large mass under the same force, which is why a rifle recoils gently while the bullet leaves at high speed.
A $0.020\ \text{kg}$ bullet is fired from a $3.0\ \text{kg}$ rifle. The expanding gas pushes the bullet forward with a force of $1800\ \text{N}$. Find the magnitude of the rifle's recoil acceleration.
In an action–reaction pair the forces are equal, but the accelerations are not: each object obeys $a = F/m$ with its own mass. A small mass accelerates far more than a large mass under the same force, the reason for gentle recoil but a fast bullet.
Add the highlighted idea to your notes before the check below.
Quick check: A truck and a car collide. The car has half the mass of the truck. Compared with the truck, the car experiences a collision force that is…
Pattern B, Identify then explain
For each example, write the action–reaction pair clearly as "X pushes Y" and "Y pushes X", and state which two objects each force acts on: (1) a person walking forward on the ground; (2) a rocket firing in space; (3) a book resting on a table. In one sentence, explain why each pair does not cancel even though the forces are equal and opposite.
Two truths, one lie: which statement about Newton’s third law is the lie?
Dot point: Use examples to describe the effects of Newton's third law of motion
Walking, swimming, rockets, all are the third law in action
We just learned that equal forces give different accelerations. That raises a question: how does this let things move forward, even a rocket with nothing to push against? This card answers it → the object pushes something backward, and the reaction pushes the object forward.
All everyday propulsion works the same way: push something backward, and by the third law it pushes you forward.
The pattern is identical across many examples:
In every case the forward motion is the reaction to something being pushed backward. A rocket does not need air to "push off", it carries its own mass (the exhaust) to throw backward.
Propulsion is Newton's third law: an object pushes mass backward (the ground, water, or exhaust gas) and the reaction force drives the object forward. A rocket works in the vacuum of space because it pushes against its own expelled gas, not against air.
Pause, copy the highlighted reasoning into your book before the check below.
Last check: Decide if the statement is true or false.
A rocket can accelerate in the vacuum of space because it pushes its exhaust gas backward and the gas pushes the rocket forward.
Fill the gaps. Third-law pairs are equal in magnitude, opposite in direction, and act on objects. They therefore never . A rocket accelerates because the exhaust gases push on the rocket.
Dot point: Use examples to describe the effects of Newton's third law of motion
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. A squid moves forward by squirting a jet of water backward out of its body. Using Newton's third law, explain how this produces forward motion, and explain why the small water jet can move the much larger squid.
1 mark: states the squid exerts a backward force on the water · 1 mark: the water exerts an equal and opposite forward force on the squid · 1 mark: the two forces act on different objects, so they do not cancel · 1 mark: the squid still accelerates because the forward reaction force is the net force acting on it (a = F/m)
UnderstandBand 5(4 marks) 2. A swimmer pushes backwards against the wall of a pool and moves forwards. Identify the third-law pair involved, and explain why these two equal and opposite forces do not cancel.
1 mark: swimmer pushes back on the wall · 1 mark: wall pushes forward on the swimmer, equal in magnitude · 1 mark: the forces act on DIFFERENT objects · 1 mark: only the force on the swimmer appears in the swimmer’s free-body diagram, so it produces their acceleration
Dot point: Use examples to describe the effects of Newton's third law of motion
ApplyBand 6(3 marks) 3. A $0.50\ \text{kg}$ ball is dropped and falls toward the Earth. Describe the third-law partner of the Earth’s gravitational pull on the ball, and explain why only the ball is seen to accelerate.
1 mark: the ball pulls the Earth upward with an equal force · 1 mark: the force on the Earth is the same magnitude, about $4.9\ \text{N}$ · 1 mark: the Earth’s enormous mass makes its acceleration immeasurably small
Dot point: Use examples to describe the effects of Newton's third law of motion
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): The squid exerts a backward force on the jet of water it expels. By Newton's third law, the water exerts an equal and opposite force on the squid, directed forward. These two forces act on different objects, one on the water, one on the squid, so they do not cancel each other. The forward reaction force is the net force on the squid, so it accelerates forward according to $a = F/m$. Even though the water is light, the equal-sized reaction force still acts on the squid and drives it forward.
Q2 (4 marks): The third-law pair is the force the swimmer exerts backwards on the wall, and the force the wall exerts forwards on the swimmer. These are equal in magnitude and opposite in direction, and both are contact forces. They do not cancel because they act on different objects: one acts on the wall and one acts on the swimmer. Forces only cancel when they are summed on the same free-body diagram, and Newton’s second law sums only the forces acting on one chosen body. On the swimmer’s diagram only the forward push from the wall appears, so the net force on the swimmer is forwards and they accelerate away. The backward force on the wall acts on the wall, which is attached to the Earth and does not measurably move.
Q3 (3 marks): The Earth pulls the ball down with a force $W = mg = 0.50 \times 9.8 = 4.9\ \text{N}$. The third-law partner is the ball pulling the Earth upward with a force of exactly $4.9\ \text{N}$. Both objects therefore experience the same size of force. They do not experience the same acceleration, because acceleration is $a = F/m$ and the two masses differ enormously. The ball, at $0.50\ \text{kg}$, accelerates at $9.8\ \text{m s}^{-2}$. The Earth, at about $6 \times 10^{24}\ \text{kg}$, accelerates at roughly $10^{-24}\ \text{m s}^{-2}$, which is far too small to detect. Equal forces do not imply equal accelerations whenever the masses are unequal.
You can now name both members of an action–reaction pair and explain everyday propulsion with the third law. Next lesson uses interacting forces to analyse connected bodies, trolleys, ropes and lifts linked together.
↩ Back to focus-area hubAt the start you predicted which vehicle feels the larger force in a car–truck crash. The answer is they feel equal forces, the third law guarantees it. The car is damaged more only because its smaller mass gives it a larger acceleration ($a = F/m$). If you said the truck pushes harder, revisit Card 2.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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